This invention relates to liquid fertilizers. More particularly, it relates to fertilizer compositions containing ureaform in suspension. These compositions have long-lasting nitrogen release characteristics. The invention also relates to processes for preparing the fertilizer compositions.
Liquid mixed fertilizers are solutions of compounds which, for the most part, are very water-soluble and can supply two or all three of the primary plant nutrients: nitrogen, phosphorus measured as P.sub.2 O.sub.5, and potash measured as K.sub.2 O. Although nitrogen-potash, nitrogen-phosphorus and nitrogen-phosphorus-potash suspensions can be made by the process of this invention, the present invention is directed primarily to suspensions containing only nitrogen.
Ureaform is a term which is used to denote mixtures of compounds of different degrees of solubility formed by the reaction of urea and formaldehyde under acid conditions, when the reaction mixture contains at least one mole of urea per mole of formaldehyde. The products of the reaction are shown below: ##STR1##
These products differ in length of chain and end group present. The methylol ureas are made primarily with a urea to formaldehyde ratio of less than one and their formation is usually catalyzed by bases. These compounds can be mixed with fillers and heated a second time to effect further polymerization and crosslinking into a resin which has no appreciable water solubility and no value as a fertilizer material because of its exceptionally slow degradation in the soil. When the urea to formaldehyde ratio is more than one, and an acid catalyst is used, methylene diurea and the related longer chain materials are formed and it is these compounds that are useful as slow-release nitrogen fertilizer materials.
The overall solubility of the ureaform material is quite low so that the product does not form highly concentrated solutions that have a tendency to burn vegetation. Moreover, the nitrogen therein becomes available as a plant nutrient over a period of time so that unusually heavy applications of the material may be made without damaging or over-feeding the plant life. The low water solubility of the ureaform fertilizer results in a high Availability Index (defined below).
For some time, it has been recognized that the measurement of the cold-water-insoluble nitrogen (CWIN) and also the measurement of that portion of the cold-water-insoluble nitrogen which is insoluble in hot water or insoluble in a buffered hot aqueous solution (HWIN), such as a hot aqueous phosphate solution, will give a good characterization of a desired fertilizer. A relationship between these two solubilities has been determined by an accepted test and is given the name of Availability Index (A.I.).
The A.I. is an empirical approximation of the water-insoluble nitrogen available for nitrification over a period of about six months, and is determined by the formula: ##EQU1## Where CWIN means cold-water-insoluble nitrogen, i.e., nitrogen that is not soluble in water at 25.degree. C..+-.2.degree. C., and HWIN means nitrogen insoluble in a hot aqueous solution of a phosphate buffer, namely KH.sub.2 PO.sub.4 and K.sub.2 HPO.sub.4, which has a pH of 7.5. The A.I. is a measure of how fast the water-insoluble nitrogen nitrifies in the soil. The larger the number, the faster the rate of nitrification.
An A.I. of 40 or greater has been considered satisfactory for urea-formaldehyde products useful in fertilizer applications, but higher values are recognized as being even better. The A.I. values for various products manufactured by typical prior art acidic processes are only in the range of 40 to 50, with the cold-water-insoluble nitrogen usually in the range of 60% to 70%.
The cold-water-insoluble nitrogen fraction is a measure of the entire amount of nitrogen available for plant nutrition over an extended period of time. It is better when it is quite high. While the A.I. indicates the relative amount of that portion of the long-lasting nitrogen which becomes available within about a six month period, it is not a precise measurement of the exact amount of nitrogen available within a six month period.
The methods for determination of the cold-water-insoluble nitrogen (CWIN) and hot-water-insoluble nitrogen (HWIN) are given in the Official Methods of Analysis of the Association of Official Analytical Chemists, Thirteenth Edition, 1980. This same publication also contains procedures for determining the other two nutrients in fertilizers; namely, potassium and phosphorus.
The production of liquid fertilizer formulations containing slow-release nitrogen has made use of several different procedures. For example, U.S. Pat. No. 3,462,256, to G. H. Justice and R. E. Formaini, describes urea-formaldehyde concentrates which are aqueous solutions containing about 80% to 90% by weight of partially reacted urea and formaldehyde in a mole ratio about 1:1 but less than 2:1. The aqueous solution is prepared by heating the urea-formaldehyde mixture with 0.3% to 6% ammonia at a temperature between 75.degree. C. and 100.degree. C. while maintaining the pH between 9.0 and 9.8 with strong alkali. Heating is continued and pH is adjusted, the patent says, until at least 50%, but no more than 80%, of the formaldehyde is in the form of methylene groups.
A different procedure involving a suspension technique is outlined in U.S. Pat. No. 3,677,736. This patent describes a liquid fertilizer suspension containing ureaform having a water-insoluble nitrogen content of at least 1.5% and an A.I. in excess of 60. The process for producing the suspension involves diluting a urea-formaldehyde reaction product having a mole ratio between 1:1 and 2:1 with water to form a 35% to 65% solution. The pH of the solution is lowered to 5, heated to a temperature range of 30.degree. C. to 80.degree. C. to form a suspension, and then the final pH is adjusted to above 5 with an alkaline material.
In addition to both the aqueous concentrates and suspension techniques, in situ urea-formaldehyde reactions have also been reported in U.S. Pat. No. 3,096,168. In this procedure, liquid mixed fertilizers containing a suspended precipitate of ureaform can be prepared by mixing specified portions of water and soluble fertilizer ingredients and reacting therein 1 to 2.5 moles of urea per mole of formaldehyde under acidic conditions to form a ureaform product.
Aqueous dispersions of urea-formaldehyde polymers have also been modified with saturated aldehydes having 2 or more carbon atoms. The high aldehydes are said to act as inhibitors of further polymerization and thus produce a stabilized ureaform material. Alcohols and sugar additives have also been used to stabilize suspensions of urea-formaldehyde polymers in addition to aqueous ammonia solutions.
Typical processes known in the art of manufacturing urea-formaldehyde fertilizers have permitted little or no control over the amount of hot-water-insoluble (HWIN) that is formed in the reaction. Also, low pH condensation conditions have generally not been utilized, since it is extremely difficult to control the rate of polymerization at pH values less than 1. At pH values greater than 1, higher carbon aldehydes, sugar and alcohols are used to stabilize the final product.
The term "stability", as applied to a liquid fertilizer material based on urea-formaldehyde condensation products, also refers generally to the maintenance of the material in a readily flowable state. Stability is usually adversely affected when the WIN is present in very fine particle size, and also when the total amount of WIN particles present is below about 10% or above about 35% by weight based on the total nitrogen present.